This present invention relates to fused silica optical members and a method of rendering the optical members resistant to compaction caused by prolonged exposure to ultraviolet laser beams.
As the energy and power output of lasers increase, the optics such as lenses, prisms, and windows which are used in conjunction with such lasers are exposed to increased irradiation levels and energies. Because of fused silica's excellent optical properties, fused silica members have become widely used as the manufacturing material for optics in such high energy laser systems.
One area of advance of such laser technology has been a move deeper into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength excimer lasers operating in the UV, deep UV (DUV), and vacuum UV (VUV) wavelength ranges. Use of such excimer laser systems is becoming popular with microlithography applications which benefit from the shortened wavelengths to increase line densities in the manufacturing of microchips. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies in the beam, each individual photon is of higher energy, irrespective of the total beam intensity. In such excimer laser systems, laser beam target areas of fused silica optics are exposed to high energy photon irradiation levels for prolonged periods of time resulting in the degradation of the optical properties of the optics.
It is known that such laser induced degradation adversely affects the optical properties and performance of the fused silica optics by decreasing light transmission levels, discoloring the glass, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Over the years, many methods have been suggested for improving the optical damage resistance of fused silica glass. It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting process, plasma CVD process, electrical fusing of quartz crystal powder, and other methods, are susceptible to laser damage to various degrees. This variable propensity to laser damage has been attributed to low OH content, sometimes measuring as low as 10 ppm or less as determined from the value of the beta-OH. As a result, the most common suggestion has been to increase the OH content of such glass to a high level. For example, Escher, G. C., KrF Laser Induced Color Centers In Commercial Fused Silicas, SPIE Vol. 998, Excimer Beam Applications, pp. 30-37 (1988), confirms that defect generation rate is dependent upon the fused silica OH content, and that "wet" silicas are the material of choice for KrF applications. Specifically, they note that high OH content silicas are more damage resistant than low OH silicas.
U.S. Pat. No. 5,086,352 and its related U.S. Pat. No. 5,325,230 have also disclosed that the ability to resist optical deterioration from exposure to a short wavelength ultraviolet laser beam depends on the OH group content in the presence of hydrogen gas. Specifically, these references show that for high purity silica glass having low OH content, KrF excimer laser durability is poor. Thus, they suggest to have an OH content of at least 50 ppm. Similarly, Yamagata, S., Improvement of Excimer Laser Durability of Silica Glass, Transactions of the Materials Research Society of Japan, Vol. 8, pp. 82-96, 1992, discloses the effect of dissolved hydrogen on fluorescence emission behavior and the degradation of transmission under irradiation of KrF excimer laser ray for high purity silica glass containing OH groups to 750 ppm by weight such as those synthesized from high purity silicon tetrachloride by the oxygen flame hydrolysis method.
Others have also suggested methods of increasing the optical durability of fused silica. For example, Faile, S. P., and Roy, D. M., Mechanism of Color Center Destruction in Hydrogen Impregnated Radiation Resistant Glasses, Materials Research Bull., Vol. 5, pp. 385-390, 1970, have disclosed that hydrogen-impregnated glasses tend to resist gamma ray-induced radiation. Japanese Patent Abstract 40-10228 discloses a process by which quartz glass article made by melting, is heated at about 400 to 1000.degree. C. in an atmosphere containing hydrogen to prevent colorization due to the influence of ionizing radiation (solarization). Similarly, Japanese Patent Abstract 39-23850 discloses that the transmittance of UV light by silica glass can be improved by heat treating the glass in a hydrogen atmosphere at 950 to 1400.degree. C. followed by heat treatment in an oxygen atmosphere at the same temperature range.
Shelby, J. E., Radiation Effects in Hydrogen-impregnated Vitreous Silica, J. Applied Physics, Vol. 50, No. 5, pp. 3702-06 (1979), suggests that irradiation of hydrogen-impregnated vitreous silica suppresses the formation of optical defects, but that hydrogen impregnation also results in the formation of large quantities of bound hydroxyl and hydride, and also results in the expansion or decrease in density of the glass.
Recently, U.S. Pat. No. 5,410,428 has disclosed a method of preventing induced optical degradation by a complicated combination of treatment processes and compositional manipulations of the fused silica members to achieve a particular hydrogen concentration and refractive index, in order to improve resistance to UV laser light degradation. It is suggested that under such UV irradiation the chemical bonding between silicon and oxygen in the network structure of the fused silica is generally broken and then rejoins with other structures resulting in an increased local density and an increased local refractive index of the fused silica at the target area. One approach which has been suggested to remedy this optical degradation problem it is to control and manipulate the chemical composition of the fused silica, and particularly the concentration of H.sub.2 dissolved in the fused silica.
More recently, U.S. Pat. No. 5,616,159 to Araujo et al., disclosed a high purity fused silica having high resistance to optical damage up to 10.sup.7 pulses (350 mJ/cm.sup.2) at the laser wavelength of 248 nm, and a method for making such glass.
While the above suggested methods are at least partially effective in reducing the absorption induced at 215 and 260 nm, there has been little or no suggestion for addressing optical damage caused by radiation-induced compaction resulting from prolonged exposure to eximer lasers. Thus, there continues to be a need for more improved fused silica glasses and methods for increasing their resistance to optical damage during prolonged exposure to ultraviolet laser radiation, in particular, resistance to optical damage associated with prolonged exposure to UV radiation caused by 193 and 248 nm excimer lasers. Accordingly, it is the object of the present invention to disclose a method of increasing the resistance of high purity fused silica glass to optical damage caused by laser induced compaction during use.